1. The Field of the Invention
The present invention relates generally to vehicle transmissions. More particularly, it concerns a continuous, variable-ratio transmission including a rotational wheel having an exterior surface of revolution defined by a tractrix curve.
2. The Background Art
Transmission devices are known in the field to convert and transmit the power generated by an engine to comply with the load placed on the output shaft. Prior art transmission devices utilize a hand-operated shift lever and foot-operated clutch to enable manually shifting among various gear ratios of the transmission. Automatic transmissions are available which do not require a clutch or shift lever. It is desirable to achieve a transmission having a continuous, infinite range of gear ratios to more efficiently respond to the various loads placed on the output shaft.
It is known in the art to provide a variable ratio transmission operating on the principle of a rotating conical surface in frictional engagement with a smaller wheel which moves along the conical surface. As the smaller wheel is moved along the conical surface, its rate of revolution relative to said conical surface changes because of the changing diameter of said conical surface. By varying the position of the smaller wheel along the conical surface, various speed ratios are thereby achieved between the two surfaces.
If the smaller wheel is provided with a cylindrical surface, shear stresses therein are unavoidable. These shear stresses can be explained by the concept that the circumference of the cylinder at its front and rear edges is the same, but these circumferences are forced to frictionally and rotatably engage with different-sized circumferences on the conical surface. The wider portions of the conical surface travel faster that the narrower portions. Since equal circumferences on the cylinder respectively engage different-sized circumferences on the conical surface necessarily travelling at different speeds, some portions of the cylinder are forced to slip and rub against the faster conical portions resulting in shear forces.
It is known to overcome this problem by providing the smaller wheel with a conical surface, instead of a cylindrical surface, in which the ratio between the contacting circumferences is the same at any point of contact between the smaller wheel and the conical surface. However, this condition will occur in only one given position. If the smaller wheel is shifted away from said given position, the ratios between the contacting circumferences will vary, causing the unwanted shear stresses.
It was discovered that such unwanted shear stresses can be substantially eliminated by changing the larger conical surface into a tractrix surface defined by a tractrix curve. The phrase "tractrix curve" as used herein refers to a curve having the property that a tangent to the curve at any point is always of equal length when measured between such point and its intersection with the X axis. In U.S. Pat. No. 3,158,041 (issued to Rae on Nov. 24, 1964), inventor Rae disclosed a tractrix surface in frictional engagement with a smaller, conical surface. However, there are many practical difficulties involved in transferring a useful amount of power from these frictionally engaging surfaces, and the prior art attempts to do so are fraught with disadvantages. For example, it is difficult to mechanically move the conical wheel along the tractrix surface and maintain contact therebetween along substantially an entire length of said conical surface. Moreover, prior art attempts to maintain such contact have required complex gear assemblies which require a larger transmission housing, and varying orientations between gear members which result in a less efficient power transfer. The prior art teaches moving the tractrix surface while maintaining a stationary conical wheel. The mechanics of doing so require more space to accommodate moving the larger tractrix surface along an arcuate movement path.